There are various known ways of controlling cooling fans for vehicle engines in order to save fuel by adapting their suction action to varying cooling requirements during operation. A commonly used method is to intermittently disconnect the driving power from the fan rotor in response to decreasing cooling requirement. Other solutions provide various ways of regulating the speed of the fan according to the cooling requirement.
A cooling fan with axially movable fan rotor is known from JP59046316 A, in which the rotor shaft is capable of linear movement against the force of a spring so that increasing engine speed causes the rotor to be drawn by its own suction force towards a constriction in the fan casing in order to increase the cooling air flow. In a cooling fan referred to in U.S. Pat. No. 4,387,780, the rotor is movable axially by a variable belt transmission upon increase in the speed of the belt transmission.